Tooltip in x/y line chart shows - and NaN

[mhkeller]

Bug Report

When creating a multi-series line chart, the values display correctly in the chart but y value shows up as NaN in a tooltip and x value is repeated and shows up as -.

Steps to Reproduce:

1. Load this dummy data of student exams in different subjects and periods: https://gist.github.com/mhkeller/8831f579f814af2a7415e043457d4e56
2. Select the X/Y line visualization
3. Group by exam_month, split by exam_subject, set the x axis to exam_month (any) and the y axis to exam_score (avg)

Expected Result:

I'd expect to see the proper x and y-value in the tooltip.

Actual Result:

I see NaN in the tooltip for exam_score and exam_month shows up twice: once as the proper value and once as -.

Environment:

2.9.0 reproduced at https://perspective.finos.org/block/?example=file

Additional Context:

[texodus]

Thanks for the report - I'm pretty sure this is the same behavior described in #2254, let me take a look and I will update